Uninterruptible power supplies (UPSs) are power conversion devices that are commonly used to provide conditioned, reliable power for computer networks, telecommunications networks, medical equipment and the like. UPSs are widely used with computers and similar computing devices, including but not limited to personal computers, workstations, mini computers, network servers, disk arrays and mainframe computers, to insure that valuable data is not lost and that the device can continue to operate notwithstanding temporary loss of an AC utility source. UPSs typically provide power to such electronic equipment from a secondary source, such as a battery, in the event that a primary alternating current (AC) utility source drops out (blackout) or fails to provide a proper voltage (brownout).
Conventional UPSs may be classified into categories. Referring to FIG. 1, a typical off-line UPS disconnects a load from a primary AC source 10 when the primary AC source fails or is operating in a degraded manner, allowing the load to be served from a secondary source such as a battery. The AC power source 10 is connected in series with a switch S.sub.1, producing an AC voltage across a load 20 when the switch S.sub.1 is closed. Energy storage is typically provided in the form of a storage capacitor C.sub.S. The secondary power source, here a battery B, is connected to the load 20 via a low voltage converter 30 and a transformer T. When the AC power source 10 fails, the switch S.sub.1 is opened, causing the load to draw power from the battery B. The low voltage converter 30 typically is an inverter that produces a quasi-square wave or sine wave voltage on a first winding L.sub.1 of the transformer T from a DC voltage produced by the battery B. The first winding L.sub.1 is coupled to a second winding L.sub.2 of the transformer T connected across the load 20. When the AC power source is operational, i.e., when the switch S.sub.1 is closed, the battery B may be charged using the low-voltage converter 30 or a separate battery charger circuit (not shown).
A line interactive (LIA) UPS topology is illustrated in FIG. 2. Here, the transformer T has a third winding L.sub.3 that may be connected in series with the load 20 using switches S.sub.2, S.sub.3 to "buck" or "boost" the voltage applied to the load 20. As with the offline UPS topology of FIG. 1, when the AC power source 10 fails, the switch S.sub.1 can be opened to allow the load 20 to run off the battery B.
As illustrated in FIG. 3, a typical on-line UPS includes a rectifier 40 that receives an AC voltage from an AC power source 10, producing a DC voltage across a storage capacitor C.sub.S at an intermediate node 45. An inverter 50 is connected between the intermediate node 45, and is operative to produce an AC voltage across a load 20 from the DC voltage. As shown, a battery B is connected to the intermediate node 45 via a DC/DC converter 60, supplying auxiliary power. Alternatively, the DC/DC converter can be eliminated and a high-voltage battery (not shown) connected directly to the intermediate node 45.
Each of these topologies may have disadvantages. For example, typical conventional on-line and LIA UPSs for 60 Hz applications use 60 Hz magnetic components (e.g., transformers and inductors) that are sized for such frequencies, and thus may be large, heavy and expensive. LIA UPSs often exhibit step voltage changes that can affect the performance of the load. Conventional off-line, LIA and on-line UPSs often use large storage capacitors, which tend to be bulky and expensive, in order to maintain an acceptable output voltage under heavy loading conditions. Moreover, because conventional UPSs are typically designed to operate in only one of the above-described off-line, LIA or on-line modes, sellers of UPSs may be required to maintain large inventories including several different types of UPSs in order to meet a variety of different customer applications.